11. ORIGIN AND AMPLIFICATION OF CLUSTER MAGNETIC FIELDS

The origin of the magnetic fields observed in galaxies and clusters of
galaxies is debated.
Very little is known about their existence before and after
the time of recombination, their evolution, and the possible impact
they could have on galaxy and structure formation.
We very briefly give the outlines of the main scenarios proposed for
the magnetic fields in the ICM, without going into
the details, which can be found in the literature
[4,
5].

According to the first scenario,
cluster magnetic fields may be primordial, i.e. generated in
the early universe prior to recombination
[3].
In this case, magnetic
fields would be already present at the onset of structure formation,
and would be a remnant of the early Universe. One mechanism for the
generation of primordial fields involves the "Biermann battery" effect
[165],
which occurs when the gradients of
electron pressure and number density are not parallel, thus
electrostatic equilibrium is no longer possible. This leads to a
thermoelectric current which generates an electric field (and a
corresponding magnetic field) that restores force balance. Other
possibilities might be that weak seed fields were formed in the phase
transitions of the early Universe, such as a quark-hadron (QCD), or
electroweak (EW) transition, where local charge separation occurs
creating local currents, or during inflation, where electromagnetic
quantum fluctuations are amplified
[166].
Values of these seed fields are of the order of ~ 10-21 G.

In principle, the presence of magnetic fields in the very early
Universe might be detectable through their effect on the Big Bang
nucleosynthesis, or if the expansion is observed to be anisotropic.
Current observations of anisotropy in the CMB place weak upper limits
(B < 5 × 10-9 G) on the
strength of a homogeneous component of a primordial magnetic field
generated in this way
[167].
By analyzing the effect of the inhomogeneities in the matter
distribution of the universe on the Faraday rotation of distant QSOs,
limits of B < 10-9 -10-8 G are obtained
[168],
depending on the assumed scales of the fluctuations.

Another scenario is that the cosmological magnetic fields are
generated in later epochs of the Universe. Gnedin et al.
[169]
argued that the strongest "Biermann battery" effects are
likely to be associated with the epoch of cosmological
reionization. Kulsrud et al.
[170]
investigated the possibility that the field may be protogalactic,
i.e. generated during the initial stages of the structure
formation process, during the protogalaxy formation.

A third scenario involves
the galactic origin, i.e. ejection from galactic winds of
normal galaxies or from active and starburst galaxies
[171,
172].
Galaxy outflows, gas stripping,
ejection from the AGN by radio jets, all contribute to deposit
magnetic fields into the ICM. Galactic fields may be arise from the
fields in the earliest stars, then ejected into the interstellar
medium by stellar outflows and supernova explosions. Alternatively,
fields in galaxies may result directly from a primordial field that is
adiabatically compressed when the protogalactic cloud collapses.
Indeed, battery mechanism on galactic scales can generate fields up to
10-19 G.

Support for a galactic injection in the ICM comes from the evidence
that a large fraction of the ICM is of galactic origin, since it
contains a significant concentration of metals. However, fields in
clusters have strengths and coherence size comparable to, and in some
cases larger than, galactic fields
[3].
Therefore, it seems quite difficult that the magnetic fields in
the ICM derive from ejection of the galactic fields. The recent
observations of strong magnetic fields in galaxy clusters suggest that
the origin of these fields may indeed be primordial.

The observed field strengths greatly exceed the amplitude of the seed
fields, or of fields injected by some mechanism. Therefore, magnetic
field amplification seems unavoidable. Dynamo effect can be at
work. A magnetic dynamo consists of electrically conducting matter
moving in a magnetic field in such a way that the induced currents
maintain and amplify the original field
[2].
The essential features of the galactic dynamo model are turbulent
motions in the interstellar medium, driven by stellar winds, supernova
explosions, and hydromagnetic instabilities.

In addition, amplification can occur during the cluster collapse.
During the hierarchical cluster formation process, mergers generate
shocks, bulk flows and turbulence within the ICM. The first two of
these processes can result in some field amplification simply through
compression. However, it is the turbulence which is the most promising
source of non-linear amplification. MHD calculations have been performed
[160,
173,
174]
to investigate the origin, distribution, strength and evolution of the
magnetic fields. The results of these simulations show that cluster
mergers can dramatically alter the local strength and structure of
cluster-wide magnetic fields, with a strong amplification of the
magnetic field intensity. The initial field
distribution at the beginning of the simulations at high redshift is
irrelevant for the final structure of the magnetic field. The final
structure is dominated only by the cluster collapse. Fields
can be amplified from
values of ~ 10-9 G to ~ 10-6 G.

Roettiger et al.
[174]
found a significant evolution (see Fig. 12) of
the structure
and strength of the magnetic fields during two distinct epochs of the
merger evolution. In the first, the field becomes quite filamentary as
a result of stretching and compression caused by shocks and bulk flows
during infall, but only minimal amplification occurs. In the second,
amplification of the field occurs more rapidly, particularly in
localized regions, as the bulk flow is replaced by turbulent motions.
Shear flows are extremely important for the amplification of the
magnetic field, while the compression of the gas is of minor
importance. Mergers change the local magnetic field strength
drastically. But also the structure of the cluster-wide field is
influenced. At early stages of the merger the filamentary structures
prevail. This structure breaks down later (~ 2-3 Gyr) and leaves
a stochastically ordered magnetic field.

Figure 12. Three-dimensional numerical MHD
simulations of magnetic field evolution in merging clusters of galaxies
174].
The evolution of gas density (column 1),
gas temperature (column 2), and magnetic pressure (column 3) in
two-dimensional slices taken through the cluster core in the plane of
the merger. The four rows represent different epoch during the
merger: t = 0,1.3, 3.4, and 5.0 Gyr, respectively.